Exposure to triclosan, an antibacterial agent used in soaps, toothpaste, and other personal care products, is widespread, with 75% of Americans having detectable levels in their bodies. In vitro and animal studies show that triclosan is an endocrine disruptor. In rodent studies, triclosan exposure during critical windows of development lowers levels of the thyroid hormone thyroxine (T4), increases allergic sensitivity and asthma-like response, decreases testosterone in males, and alters pubertal timing in females. To date, only a small number of studies have examined health effects of triclosan in humans, focusing on school-age children and adults. These studies provide some confirmation of effects on endocrine and immune systems, but they do not address exposure in utero - a critical period for endocrine disruption and immune development - and thus likely underestimate triclosan's potential health impact. Both the U.S. Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) are embarking on comprehensive regulatory reviews of triclosan, but are challenged by a dearth of human data. High-quality, independent research assessing associations of prenatal and early childhood triclosan exposures with subsequent health effects are urgently needed. The Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) is a longitudinal birth cohort study that has gathered comprehensive data on environmental exposures, health, and development in more than 300 children who have participated from before birth through age 12. We propose to characterize urinary triclosan concentrations from CHAMACOS mothers during pregnancy and their children at 5 and 9 years of age and to examine the potential impact of this exposure on: 1) thyroid hormone homeostasis in pregnant women and newborns, 2) allergy and asthma symptoms in children at ages 5 and 7, and 3) timing of puberty and sex hormone levels in children between ages 9 and 12. We have already collected most of the data needed to achieve these aims, and crude, preliminary analyses suggest associations of triclosan with decreased T4 and increased risk of asthma symptoms in this cohort. However, the work needed to finalize urinary triclosan measurements, obtain additional outcome data, and conduct sophisticated data analyses that control for longitudinal effects and confounding cannot be completed without dedicated funding. This study is exploratory, because it is the first longitudinal study of developmental triclosan exposure; developmental, in that it lays groundwork for future in-depth research; and high risk, because we hope to have results in time to inform pending policy decisions. By relying on a rich existing data, this project can be completed in just two years, promising high regulatory and scientific impact.